This invention relates to a gravity separation process for removing tar from water in a condensate formed by cooling of a raw gas produced by the gasification or dry distillation of solid fuels, particularly coal.
The cooling of gas produced by the gasification or dry distillation of coal results in the formation of a condensate containing water and tarry constituents. The water (sometimes referred to as gas-water)contains water-soluble products of the dry distillation of coal, such as phenols and fatty acids, and also dissolved gases, mainly CO.sub.2 and NH.sub.3, and products which are not soluble in water, such as tar, tar oil, gasoline or petrol from gas, and coal dust and ash dust.
The gasification of coal under normal pressure or superatmospheric pressure by a treatment with oxygen and water vapor and, if desired, other gasifying agents, such as CO.sub.2, is known, e.g., from the Printed German Application 1,021,116. The dry distillation of coal to produce town gas is also a known process.
The raw gas produced by the gasification and dry distillation of solid fuels, particularly coal, is cooled before it is further used. The cooling results in the condensation of water vapors and of products of the dry distillation of coal, and part of the dry distillation products as well as entrained dust and other inorganic products are scrubbed off by the condensation which accompanies the cooling.
The cooling and the accompanying condensation result in a condensate which contains a proportionally large amount of H.sub.2 O and also contains the products of the dry distillation of coal. These dry distillation products consist mainly of tar, which is composed of various compounds. The latter may be divided into three fractions, the first of which contains the high-boiling constituents which have higher specific gravity than water. The lower-boiling, so-called tar oil fraction has a lower specific gravity than water. The third fraction is the so-called gasoline or petrol from gas and differs from the tar oil by an even lower specific gravity and from tar and tar oil in that it has only a very low content of oxygen compounds.
When it is desired to subject the two heaviest fractions together with the gas-water to a separation of tar and water, difficulties may arise because the average specific gravity of tar and tar oil is close to the specific gravity of water. For this reason it may be more desirable to cool the gas in several stages, in a plurality of heat exchangers connected in series. This fractional distillation of tar and water and the scrubbing of other substances from the raw gas affords the further advantage that an economical recovery of water is enabled and waste heat at temperatures above 100.degree. C can be used to produce steam in the first cooling stages.
The tar-containing aqueous condensate which becomes available in the various cooling and condensing stages used for the fractional condensation is fed to at least two different gravity separators. One separator serves to remove water and higher-boiling tar fractions. It is supplied with the condensate from the first cooling stages. This condensate is hotter than the condensate of the succeeding cooling stages and is generally at a temperature above 100.degree. C. A second separator separates water, which has formed in the succeeding cooling stages, from the lower-boiling tar fractions, which are generally called tar oil.
It has been found, however, that it is difficult to separate the tar and water coming from the first cooling stages because tar-water emulsions may form as a result of an interaction of substances which are contained in the raw gas and are contained in the water after the first scrubbing and condensing stage. The substances having the emulsifying activity are organic and inorganic substances. It is known that ash which has been entrained by the gas and scrubbed off in the first cooling stage can promote the formation of emulsion, particularly if the ash contains lime and lime phenolates may form. There are some other mechanisms which may promote an emulsion.
Emulsions may be broken by demulsifiers. Calcium chloride, e.g., has been used with good success. A disadvantage resides in the costs involved and in the additional pollution of the water with inorganic substances.
The pH-value of the water is generally above 8. Emulsions of water and tar can be broken by a reduction of the pH-value, i.e., by an acidification. This measure is not feasible in practice in view of the high rates at which gas water becomes available in the beneficiation of coal.